This invention relates to an ion implantation system having an angular energy filter that deflects an ion beam by the use of at least one of an electric field and a magnetic field and further relates to a beam space-charge compensation device suitable for application thereto.
An ion implantation system is widely used in the process of manufacturing semiconductor integrated circuits because of its capability of introducing impurities into fine regions of a surface of a processing object with high accuracy. In the ion implantation system, since ions having charges are implanted into a wafer as a processing object, the storage of charge (charge-up) onto the wafer becomes a problem. In addition, there also arises a problem of divergence of an ion beam due to space charge produced in a beam line.
Since the ions that are implanted normally have positive charges, negative charges (electrons) are supplied for defusing the charge-up and suppressing the ion beam divergence. As an example thereof, there is a method of positively supplying electrons produced by collision of the ions with walls of the beam line. Alternatively, there is a method of producing secondary electrons by the use of an electron gun near the wafer and supplying them. Among various methods like these, use is widely made of a plasma shower (or plasma flood gun) as a method that can supply relatively low-energy electrons.
In a batch ion implantation system, wafers are mounted on a rotary disk capable of a linear reciprocating motion to thereby enable ion implantation over the whole surface of each wafer. In this case, a trajectory of an ion beam is fixed with respect to a beam line. A plasma shower is provided near the trajectory of the ion beam so that electrons are extracted from the plasma shower by the potential of the ion beam.
FIG. 1 is an exemplary diagram of a conventional plasma shower used in a batch ion implantation system for defusing the charge-up.
In FIG. 1, a plasma forming gas 216 is introduced into an arc chamber 215. Plasma is formed by heating a filament 217 using the power from a power supply 218 while applying an arc voltage 219 across the arc chamber 215 and the filament 217. By configuring that an ion beam 228 is located near the arc chamber 215, electrons are extracted due to the potential generated by the ion beam 228 so that the charge-up caused by the ion beam 228 is suppressed. It is assumed that the ion beam 228 is advancing from the front side toward the back side of the sheet of FIG. 1. Feeding of electrons from the arc chamber 215 to the ion beam 228 can be facilitated by disposing a shower tube 237 so as to surround the ion beam 228 and applying a potential 238 to the shower tube 237.
On the other hand, in an ion implantation system having a deflection mechanism for scanning that carries out beam deflecting for scanning by providing a linear reciprocating motion of an ion beam itself, the relative position between the ion beam and a plasma shower constantly changes and therefore a stable supply of electrons is difficult in view of this, there are proposed various methods for supplying electrons extracted from a plasma shower to an ion beam that is deflected for scanning.
As an example, there is proposed a method of applying a magnetic field over a deflecting area of an ion beam, of which the trajectory changes in a wide range, in order to facilitate feeding of electrons to the ion beam in an ion beam charge compensation device (e.g. JP-A-H09-147785). In this method, a plasma arc chamber is disposed at the center of the beam deflecting area so as to be perpendicular to the beam and the magnetic field spreading from the center of the plasma arc chamber over the whole beam deflecting area is generated by a coil.
However, in the method of spreading the electrons extracted from the single portion over the deflecting area of the ion beam by the use of the magnetic field, since a leakage magnetic field exists in a beam line, the ion beam is bent so that distribution and implantation angle of the ion beam are adversely affected.
The following methods are employed in the conventional plasma shower.
(a) Since electrons move so as to wind around lines of magnetic force, when there is a magnetic field around the plasma shower that precludes extraction of electrons or feeding of electrons to an ion beam, a magnetic shield is provided to thereby reduce the magnetic field.
(b) By providing a magnetic field generating portion, a magnetic field is generated which serves to increase the efficiency of plasma production in the arc chamber or the efficiency of electron extraction from the arc chamber.
(c) A magnetic field generating portion is further provided in the beam line to thereby generate a magnetic field that efficiently confines the electrons extracted into the beam line.
In addition, in the ion implantation system, a deflection portion called an angular energy filter (hereinafter abbreviated as an “AEF”) is normally provided on the downstream side of the foregoing deflection mechanism for scanning. As will be described later, the AEF has an AEF chamber. In the AEF chamber, an electric field or a magnetic field (hereinafter referred to as an “AEF magnetic field”), which is strong, is generated for bending the ion beam.
Assuming that the plasma shower is arranged in the AEF chamber for producing plasma in the presence of the AEF magnetic field, it is necessary to provide a mechanism for canceling the AEF magnetic field like in the foregoing method (a) or a mechanism for generating a magnetic field effective for the plasma shower like in the foregoing method (b) or (c). In any event, however, it is not desirable to provide the new mechanism in terms of concern about complexity of the ion implantation system, disturbance of the AEF magnetic field, or the like.